1. Field of the Invention
This application relates to an improved process for the preparation of neopentyl glycol polyesters and co-polyesters preferably having average molecular weights in the range of from about 400 to 6,000.
2. Description of the Prior Art
Neopentyl glycol polyesters and co-polyesters are known. Polyurethanes produced from them are distinguished by their optimum mechanical properties such as advantageous cold characteristics and exceptional resistance to hydrolysis, which are important, for example, in coatings of all types.
Neopentyl glycol polyesters and co-polyesters are normally prepared by the solvent-free esterification of (a) polyvalent and optionally also monovalent, preferably divalent carboxylic acids on their anhydrides or esters with low-boiling monohydric alcohols: (b) neopentyl glycol; and optionally (c) additional polyhydric alcohols, in particular higher boiling diols (Houben-Weyl, Methoden der Organischen Chemie, Volume 14/2, page 1 et seq, 1963). In such solvent-free esterification processes, the major portion of the water of condensation or of the monohydric alcohol is first distilled off after the starting components have been mixed, and esterification is then completed under vacuum, with removal of the remainder of the water of condensation or monohydric alcohol. It is well known that when the process is used for the preparation of neopentyl glycol polyesters or co-polyesters, technical difficulties arise at this stage, due to the physical properties of neopentyl glycol, since it has a melting point of about 127.degree. C. and a boiling point at atmospheric pressure of about 208.degree. C., but at pressures below 50 mbar, neopentyl glycol boils in the region of, or below, its melting point, i.e., it sublimes. Since the temperature in the reaction vessel during the vacuum phase of the preparation of neopentyl glycol polyesters is approximately from 200.degree. to 220.degree. C., it is found that even at a comparatively slight vacuum neopentyl glycol enters the column in considerable quantities and immediately crystallizes there. This phenomenon leads to several problems. Firstly, esterification is accompanied by an uncontrolled lowering of the concentration of neopentyl glycol in the reaction vessel due to sublimation losses. This cause an undesirable change in the prescribed molar ratio of acid to alcohol groups. In the course of condensation, part of the sublimating neopentyl glycol is deposited in those parts of the apparatus where it is liable to cause blockages, so that the preparation of neopentyl glycol polyesters is also a problem from a safety point of view. Added to this are the problems of environmental contamination, in particular, the contamination of effluent by neopentyl glycol.
It is known that substantial quantities of glycols which have a higher boiling point than neopentyl glycol may be used in the preparation of neopentyl glycol polyesters without impairing the properties of the synthetic materials produced from the polyesters. Co-polyesters of, for example, adipic acid, neopentyl glycol and hexanediol are therefore particularly important. If, however, substantial quantities of lower boiling glycols are used (e.g., ethylene glycol), there is an undesirable loss in the resistance to hydrolysis of the end products.
It was therefore an object of the present invention to provide a process which could easily be carried out for the preparation of neopentyl glycol polyesters and co-polyesters preferably having average molecular weights of from about 400 to 6,000 without the above-mentioned disadvantages of the known art.
This problem could surprisingly be solved by using certain low-boiling glycols described in more detail below at certain concentrations described below in the preparation of neopentyl glycol polyesters and co-polyesters.